I've been following Parse for a while now, and I agree with technogarden. Their platform is impressive, and they're adding useful new features at a rapid rate.
It would definitely be cool if they open-sourced the platform, but, in the meantime, you might be interested in this project:
Personally, though, I don't mind paying for a service like Parse. If someone is willing to not only build the features I need but also to wear a pager and get up at 3am just to keep my users happy, I'm more than happy to give them some of my money.
Same here, happy to give money to make a pain point go away so I can focus on the product itself.
My only wish was that I could place an easy API layer between Parse and my app, because I would want more fine grained control over how data is mutated (even with ACLs, someone with write permission could mutate data and corrupt it if desired).
There's a REST API, but I don't think I would want to use that as much.
Check out Weary (https://github.com/mwunsch/weary/wiki). It's a framework for building REST clients in Ruby. Clients written with Weary are also Rack apps which can can be mounted in a Rails app, for example. Then you can add your own layer of authentication/whatever to the Rails app which will act as a proxy to your Parse db.
It would definitely be cool if they open-sourced the platform, but, in the meantime, you might be interested in this project:
http://code.google.com/p/openmobster/wiki/learnmore
Personally, though, I don't mind paying for a service like Parse. If someone is willing to not only build the features I need but also to wear a pager and get up at 3am just to keep my users happy, I'm more than happy to give them some of my money.